Method for correcting the combustion settings of a set of combustion chambers and apparatus implementing the method

ABSTRACT

The invention relates to a method for real-time, continuous adjustment of a set of thermal combustion chambers ( 2, 2   i ) having similar characteristics supplied by the same combustive agent ( 8 ) and fuel ( 9 ) networks, according to which one of the chambers ( 2   i ) is used as a reference chamber, combustion parameters are measured ( 7 ) in the fumes output from the reference chamber, and the control parameters of all the chambers ( 2 ) are adjusted according to the combustion parameters measured at the output of the reference chamber ( 2   i ), in particular according to the ratio of combustive agent to fuel and/or the composition of the combustive agent and/or fuel.

The present invention relates to a method for the adjustment of devicesfor supplying a set of combustion chambers supplied with a combustionagent and a fuel, the properties thereof being variable. The inventionenables possible fluctuations of said properties to be taken intoaccount so as to maintain the desired quality of combustion.

Numerous industrial heating devices consist of a set of separatecombustion chambers supplied by a common device for supplying combustionagent and fuel. For example, in the iron and steel industry the devicesare furnaces for continuous processing lines for metal strips, providedwith radiant tubes.

In many cases, the combustion agents or fuels used have properties whichmay vary over time. For example, when using enriched air or depletedair, or even when using fuels co-produced on site such as coke oven gas,steel furnace gas, a mixture of gases or even alternative fuels such asbiogas or generator gas.

Numerous methods exist for correcting the regulation of a furnaceaccording to the properties of the fuel. Said methods make it possibleto correct the fluctuation of one of the principal parameters of thefuel, for example the calorific power PCI, the combustive power, thecombustion index or Wobbe index.

Said methods do not permit the properties of the fuel and the combustionagent to be taken properly into account as, in the majority of cases,the sampling process modifies the properties of the fuel and thecombustion agent. Said methods also have the drawback of having arelatively long response time and require devices for measuring inprotected areas to be put in place.

A specific method is proposed by FR2712961. According to this method, aburner is controlled by removing a given quantity of fuel, said fuelbeing combusted in a dedicated chamber with a quantity of air resultingin combustion in excess air, and a variable reflecting the deviationfrom stoechiometry is measured in the fumes from the complete combustionof the fuel. This variable is then used for determining therepresentative value of the coefficient for regulating the quantity ofcombustion air from the burner. Said device for measuring the combustionconsists of a combustion tunnel, a burner and appropriate measuringmeans.

Said system has the drawback of using a specifically constructedmini-furnace in which an air/fuel mixture is combusted in proportionswhich are controlled in order to provide an excess level of air. Saidmini-furnace consumes fuel and has to operate with an excess level ofair. The combustion is carried out in said mini-furnace at an operatingpoint which may be very different from that of the apparatus to beregulated, for example in terms of the power of the burner, the air/gasratio, the temperature of the furnace and the containment of the flame.The processing of the results thus requires the use of mathematicalcorrection formulae.

Said system also has the drawback of being exclusively dedicated totaking into account variations in the properties of the fuel. It doesnot take into account variations in the properties of the combustionagent nor the combustion parameters appropriate for the monitoredapparatus, namely the proportion of unburnt residues or even theproportion of nitrogen oxide NOx emissions.

The object of the invention is primarily to improve the control of thecombustion of a set of combustion chambers involved in a manufacturingprocess; in particular, the invention aims to improve the control of aset of radiant tubes for a continuous processing line for metal strips.

Said combustion chambers are of the same type. The power of each of saidcombustion chambers may be variable but the technologies implemented aresimilar. The potential differences between said combustion chambers donot influence the combustion parameters which are desired to bemonitored.

In order to ensure the control of the combustion of the set of radianttubes, one of said tubes installed in the furnace shell is used as areference tube. Said reference tube takes part in the heating of thestrip, as do all the other tubes. Said radiant tube has a similarconstruction to that of the other tubes and is acted upon in a mannerrepresentative of the operation of the other tubes which take part inthe heating process.

The combustion products at the outlet of the radiant tube are cooled bytaking part in the process of heating the furnace. A measurement deviceis installed at the outlet of said reference tube, said measurementdevice providing information about the status of the combustion productsdownstream of the tube. Generally, said device is a gas analyzer whichpermits information, for example about the excess air and/or unburntresidues, to be obtained, in particular.

Thus, according to the invention, the method for real-time andcontinuous control of a set of thermal combustion chambers havingsimilar characteristics, supplied by the same combustion agent and fuelnetworks, is characterized in that:

-   -   one of the chambers is used as reference chamber,    -   combustion parameters are measured in the fumes at the outlet of        the reference chamber,    -   the control parameters of all the chambers are adjusted        according to the combustion parameters measured at the outlet of        the reference chamber, in particular according to the ratio of        combustion agent to fuel and/or the composition of the        combustion agent and/or fuel.

The combustion parameters measured in the fumes at the outlet of thereference chamber may consist at least of the concentration of oxygenand/or the concentration of carbon dioxide and/or the concentration ofhydrogen and/or the concentration of nitrogen oxides and/or the rate ofunburnt residues.

The level of excess air may be determined according to the concentrationof oxygen or carbon dioxide in the fumes at the outlet of the referencechamber. The lack of air may be determined by the concentration ofcarbon monoxide or hydrogen or unburnt residues in the fumes at theoutlet of the reference chamber.

Advantageously, the control parameters for all the chambers are adjustedso as to obtain the desired level of excess air and/or rate of unburntresidues. It is possible to adjust the control parameters for all thechambers so as to obtain the desired lack of air and/or the rate ofunburnt residues.

The concentration of nitrogen oxides may be regulated by adjusting thecomposition of the combustion agent and/or fuel, in particular bydiluting with combustion products.

The thermal chambers may consist of radiant tubes supplied withproportions of gas which are different from those of the reference tube.

The invention also relates to an apparatus for implementing a method asdefined above, comprising a set of thermal combustion chambers havingsimilar characteristics, supplied by the same combustion agent and fuelnetworks, characterized in that:

-   -   one of the chambers is used as a reference chamber,    -   means for measuring the combustion parameters are installed at        the outlet of the reference chamber to measure said combustion        parameters in the fumes at the outlet,    -   control means are provided to adjust the control parameters of        the set of chambers according to the measures implemented at the        outlet of the reference chamber, in particular according to the        ratio of combustion agent to fuel and/or the composition of        combustion agent and/or fuel.

The thermal chambers may consist of radiant tubes and a reference tubemay be supplied together with the other tubes, the control members beingplaced on supply circuits common to all the tubes.

According to a further possibility, the radiant tubes are suppliedseparately from the reference tube and have different control membersfrom those of the reference tube, on different supply circuits, thecontrol members of the tubes being controlled such that said tubes aresupplied with the same proportions of gas as the reference tube.According to a further possibility, the control members of the radianttubes may be controlled such that said tubes are supplied withproportions of gas which are different from those of the reference tube,in particular with a different level of excess air.

Relative to the equipment used in the prior art, according to theinvention a standard device is used as the combustion chamber, saiddevice being similar to the others and being used in the method, and thecontrol thereof being entirely representative of the operation of allthe devices. In particular, it permits the parameters which depend onthe mode of operation of the entire apparatus to be controlled.

Generally, the main parameter is the level of excess air measured by theconcentration of oxygen. However, it may be necessary to monitor otherparameters, for example the unburnt residues when the furnace is cold oreven the nitrogen oxide emissions which may be regulated, for exampleaccording to variations in the properties of the combustion agent and/orfuel.

The method according to the invention also enables devices functioningin the absence of air to be regulated. In this case, the monitoredparameter may be a component representing one of the gasesrepresentative of this combustion mode, present in a high proportion inthe fumes. This gas may, for example, be carbon monoxide CO.

The reference tube is supplied by circuits supplying combustion agentand fuel provided with control members enabling the proportion thereofto be adjusted. Said reference tube may also be supplied by additionalcircuits, for example for combustion fumes or oxygen.

The analyses carried out on the combustion products at the outlet of thereference tube are taken into account to control the control members ofthe supply circuits so as to obtain the desired quality of combustion.

When the reference tube is supplied together with the other tubes, i.e.when the control members are placed on supply circuits common to all thetubes, including the reference tube, the other tubes benefit from thissame control.

When the radiant tubes are supplied separately from the reference tube,i.e. they have control members on the different supply circuits, saidcontrol members are controlled such that said tubes are supplied withthe same proportions of gas as the reference tube, or with a differentproportion.

If the two sets of radiant tubes are designed to have identicalcontrolled parameters, for example an identical supply pressure for adifferent operating power, whilst maintaining the same proportion ofgas, the controlled parameters of the reference tube are reproduced forthe members of the supply circuits of the second set of tubes.

If the two sets of radiant tubes are designed to have differentcontrolled parameters, for example a different supply pressure, whilstmaintaining the same proportion of gas, the controlled parameters of thereference tube are corrected for the members of the supply circuits ofthe second set of tubes. Said correction depends on the law of physicsof the measured value, for example the variation in the flow rateaccording to the variation using the square of the differentialpressure.

According to the invention, when the radiant tubes are suppliedseparately from the reference tube, their control members may becontrolled such that said tubes are supplied with different proportionsof gas from those of the reference tube, for example with a differentlevel of excess air.

The control of the calorific requirement of the reference tube may beimplemented by altering the flow rate or altering the duration. Thecontrol may be associated with that of other radiant tubes or it may beseparate.

Apart from the arrangements set forth above, the invention consists of acertain number of other arrangements which will be referred to in moredetail hereinafter with reference to embodiments, disclosed withreference to the accompanying drawings, but which are in no waylimiting. In the drawings:

FIG. 1 is a schematic view of a continuous treatment line for metalstrips,

FIG. 2 is a schematic view of a radiant tube,

FIG. 3 shows an example for supplying a pair of radiant tubes,

and FIG. 4 is a second example for supplying a set of radiant tubes.

With reference to FIG. 1 of the drawings it is possible to see a furnace1, shown schematically, provided with radiant tubes 2 providing theheating of a metal strip 3, passing along rollers, not shown. Theheating is carried out in a protective atmosphere, generally composed ofa mixture of nitrogen oxide and hydrogen. In each radiant tube, thecombustion is ensured in a confined combustion chamber, separated fromthe furnace shell by the tube. Thus indirect heating takes place.

With reference to FIG. 2 of the drawings, it is possible to see aU-shaped radiant tube 2, shown schematically. The radiant tubes may haveother shapes, for example they may be I-shaped, P-shaped, doubleP-shaped or W-shaped. A burner 4 is located at one end of the tube 2.The flame 5 is propagated in the tube and releases its energy toward thewalls of the tube which heats the shell and the strip. As they pass intothe tube, the fumes thermally dissipate on the walls of the tube. A heatrecovery system, not shown, enabling the combustion air to be preheatedis generally positioned at the end of the tube opposing the burner. Atthe outlet of the tube, the partially dissipated fumes are evacuatedtoward a collector 6 connected to a flue. An analyzer 7 positioneddownstream of the outlet of the tube, permits the analysis of thecombustion products.

With reference to FIG. 3 of the drawings, it is possible to see anembodiment of the circuit for supplying combustion agent and fuel to aset of U-shaped radiant tubes, including a reference tube 2 r, shownschematically. The burners 4 are supplied by a system for supplyingcombustion agent 8, with a control valve FCVc and a system for supplyingfuel 9 with a control valve FCVf. The automatic control of the valvesmay be ensured by an automatic system (not shown) which receives at theinlet information about the combustion parameters measured by theanalyzer 7 at the outlet of the reference tube 2 r. The automated systemprovides, at the outlets connected to the controls of the valves,specific instructions for control as a function of the input data.

In this embodiment, the furnace zone is provided with radiant tubes 2,the burners 4 thereof being supplied in parallel by common combustionagent and fuel networks. The variation in the calorific requirement iscontrolled by the operating times of the burners, i.e. for each burnerthe proportion of the duration of opening of its combustion agent valveFCVc and its fuel valve FCVf during a given time.

Each of the burners has been previously controlled during commissioningfor a known operating point by means of manual flow limiters 10. For agiven operating point, for example at 100% power, each of the burnershas been previously controlled by an individual flow limiter device. Inthese conditions, for a given general operation of the furnace zone, theoperation of a tube selected as a reference tube generally illustratesthe operation of the set of burners connected to the same supplysystems. By modifying the settings of the system for supplying thereference tube, i.e. the valves FCVc and FCVf, to take into account thechange in the characteristics of the fuel or combustion agent, thecontrol of all the radiant tubes is modified.

The control of the valves FCVc and FCVf causes a variation in thepressure of the combustion agent Pc or the pressure of the fuel Pf,enabling the proportion of the gases to be adjusted to be identical withall the tubes.

The measurements carried out by the analyzer 7 placed at the outlet ofthe reference tube may, for example, be limited to two parameters: theoxygen content which represents the level of excess combustion agent andthe proportion of unburnt residues, the quantity thereof resulting incorrections to the level of excess air, in particular at lowtemperatures.

With reference to FIG. 4 of the drawings, it is possible to see afurnace provided with U-shaped radiant tubes, shown schematically.

In this apparatus, a set of four radiant tubes 2 is supplied bycombustion agent 8 and fuel 9 networks common to the set of four tubesby means of the control valves FCVc and FCVf. In this apparatus, areference radiant tube 2 i is supplied separately by its own system forsupplying combustion agent 8 i regulated by a valve FCVci and fuel 9 iregulated by a valve FCVfi.

According to a preferred embodiment of the invention, the transmittersused on the reference tube 2 i and the set of tubes 2 to be regulated,make use of the same laws of physics. For example, it is possible to usea differential pressure transmitter to measure the flow rate.

At a given operating point, for example at 100% power, each of theburners, including that of the reference tube, has been previouslycontrolled during commissioning by a separate flow limiter device 10, inparticular by manual control, so as to obtain the same ratio ofcombustion agent/fuel at each burner. This control is carried out in thesame conditions for all the tubes, i.e. with the same quality ofcombustion agent and fuel.

It should be noted that each of the tubes may have a nominal powerand/or different dimensions. The tubes may also be of different shapes,for example they may be U-shaped or W-shaped.

In control mode, the flows of combustion agent and fuel from thereference tube are adjusted so as to have the correct rate of oxygen inthe fumes according to variations in the properties of the fuel and/orthe combustion agent. The analyzer 7 enables the valves FCVci and FCVfito be controlled so as to obtain the desired combustion quality in thereference tube. The corresponding operating point measured by thepressures Pci and Pfi defines the control variables Pc and Pf used tocontrol the valves FCVc and FCVf.

As the settings of the reference tube 2 i are representative of thedesired combustion at all the tubes, the values measured at its supplysystem are used to govern the regulation of the set of tubes 2.

For example, the fuel valve FCVf will be controlled according to thecalorific requirement and the combustion agent valve FCVc will becontrolled so that the value Iva of the measurement signal which isrepresentative of the flow of combustion agent at the set of tubes to beregulated is defined according to the following relation:

Iva/Ivf=K×Ivai/Ivfi

In this relation, K is a constant value, Ivai expresses the measurementsignal which is representative of the combustion agent at the referencetube 2 i, Ivfi expresses the signal of the measurement which isrepresentative of the fuel at the reference tube 2 i and Ivf expressesthe measurement signal which is representative of the fuel at the set oftubes 2 to be regulated.

The advantage of this system is that it enables corrections to be easilymade to the regulation of a heating system. The response time is thusvery short and the control parameters are entirely representative of thedesired control.

An extension of this application is to control an apparatus of which thecomposition of the combustion agent is variable. This variation may beunintentional, for example the composition of the air is dependent onthe humidity content.

This variation may be intentional, for example by modifying the rate ofoxygen of the combustion agent. Thus, oxygen enrichment may be carriedout to increase the output of a furnace, reduce the consumption of fuelor reduce CO2 emissions. Oxygen depletion may be implemented in order tomodify the thermal transfer, for example by extending the flame, or toreduce the NOx emissions. In this application, the measurement of NOx inthe fumes of the reference tube serves to regulate the rate of dilutionof the combustion agent.

The invention makes it possible to adjust the settings which may bedifferent from those of the reference burner. According to theinvention, the flame develops in a reference chamber which is similar tothe other chambers, but not in the open air.

The combustion in a chamber is significantly influenced by the geometrythereof. Said geometry dictates the containment of the flame, the natureof the flow of gas, the recirculation of part of the fumes and thetemperature cartography in the chamber. All these parameters influencethe combustion, in particular the temperature of the flame.

The results of the combustion measured at the outlet of the referencechamber are thus directly representative of the combustion as producedin the other chambers.

1-12. (canceled)
 13. A method for real-time and continuous control of aset of thermal combustion chambers (2, 2 r, 2 i) having similarcharacteristics, supplied by the same combustion agent (8) and fuel (9)networks, the method comprising: using one of the chambers (2, 2 r, 2 i)as a reference chamber; measuring combustion parameters (7) in the fumesat an outlet of the reference chamber; and adjusting control parametersof all the chambers according to the combustion parameters measured atthe outlet of the reference chamber, in particular according to theratio of combustion agent to fuel and/or the composition of combustionagent and/or fuel.
 14. The method as claimed in claim 13, wherein thecombustion parameters measured in the fumes at the outlet of thereference chamber include at least a concentration of oxygen and/or aconcentration of carbon dioxide and/or a concentration of carbonmonoxide and/or a concentration of hydrogen and/or a rate of unburntresidues and/or a concentration of nitrogen oxides.
 15. The method asclaimed in claim 14, wherein the level of excess air is determinedaccording to the concentration of oxygen or carbon dioxide in the fumesat the outlet of the reference chamber.
 16. The method as claimed inclaim 14, wherein the lack of air is determined by the concentration ofcarbon monoxide or hydrogen or unburnt residues in the fumes at theoutlet of the reference chamber.
 17. The method as claimed in claim 13,wherein the control parameters of all the chambers are adjusted so as toobtain the desired level of excess air and/or rate of unburnt residues.18. The method as claimed in claim 16, wherein the control parameters ofall the chambers are adjusted so as to obtain a desired lack of air. 19.The method as claimed in claim 14, wherein the concentration of nitrogenoxides is regulated by adjusting the composition of the combustion agentand/or fuel, in particular by diluting with combustion products.
 20. Themethod as claimed in claim 13, wherein the thermal chambers compriseradiant tubes, one of the radiant tubes being a reference tube, and theother radiant tubes being supplied with proportions of gas which aredifferent from those of the reference tube.
 21. An apparatus forimplementing a method as claimed in claim 13, comprising a set ofthermal combustion chambers (2, 2 r, 2 i) having similarcharacteristics, supplied by the same combustion agent (8) and fuel (9)networks, wherein: one of the chambers (2, 2 r, 2 i) is used as areference chamber, means for measuring (7) the combustion parameters areinstalled at the outlet of the reference chamber to measure saidcombustion parameters in the fumes at the outlet, control means (FCVc,FCVf) are provided to adjust the control parameters of the set ofchambers according to the measures implemented at the outlet of thereference chamber, in particular according to the ratio of combustionagent to fuel and/or the composition of combustion agent and/or fuel.22. The apparatus as claimed in claim 21, wherein the thermal chamberscomprise radiant tubes, wherein a reference tube (2 r) is suppliedtogether with the other tubes, the control members (FCVc, FCVf) beingplaced on supply circuits (8, 9) common to all the tubes.
 23. Theapparatus as claimed in claim 21, wherein the thermal chambers compriseradiant tubes, wherein one of the radiant tubes is a reference tube (2i), and wherein the other radiant tubes are supplied separately from thereference tube (2 i) and have different control members (FCVc, FCVf)from those (FCVci, FCVfi) of the reference tube (2 i), on differentsupply circuits (8, 9, 8 i, 9 i), the control members (FCVc, FCVf) ofthe tubes (2) being controlled such that said tubes (2) are suppliedwith the same proportions of gas as the reference tube (2 i).
 24. Theapparatus as claimed in claim 23, wherein the control members (FCVc,FCVf) of the radiant tubes (2) are controlled such that said tubes aresupplied with proportions of gas which are different from those of thereference tube (2 i), in particular with a different level of excessair.